1. Field of the Invention
This invention relates to an assembly for the installation of two coaxial shafts, particularly a so-called low pressure shaft as shown in FIG. 1, connecting the low pressure compressor 1 to the low pressure turbine 2 in a gas turbine, surrounded by a high pressure shaft connecting the high pressure compressor 3 to the high pressure turbine 4. Throughout this specification, the front part of the machine is at the left-hand part of the drawings and the rear part of the machine is at the right-hand part.
2. Description of Related Art
In this type of machine, the internal shaft 15 (low pressure) projects beyond the front of the external shaft 16 (high pressure) and can finish at a shaft end 5 supporting a large volume fan 6 on which large radius blades are fitted to discharge air into an auxiliary stream 7 surrounding the main gas stream 8. If there is an accidental breakage in this fan, there will be a severe out-of-balance mass on the internal shaft 15. Thus, some arrangements need to be made. Firstly, a bearing 9, also referred to as the number 1 bearing, right at the front of the internal shaft 15, may be made fusible or breakable; in other words, it may be installed on a low resistance support 10 that is designed to break as soon as an out-of-balance mass develops, so that the resulting large forces that are then generated are not transmitted to the rest of the stator structure. The shaft end 5 is then free to tilt by moving under the effect of the out-of-balance mass until the assembly that it forms with the fan 6 moves into a new equilibrium position in rotation, without necessarily causing any further damage until the machine stops. The shaft end 5 tilts around another bearing 17, also referred to as bearing number 2, behind the previous bearing 9, that supports the back end of the shaft end 5 and a main portion of the internal shaft 15. This bearing 17 controls the axial position of the internal shaft 15.
A take-off gear 20, designed particularly to start the machine and activated by a transmission shaft 25 perpendicular to the shafts 15 and 16, must be placed between this bearing 17 and still another bearing 18, also referred to as bearing number 3, that provides support to the forward end of the external shaft 16. The bearing 17, being as far backwards as possible to improve the dynamic resistance of the internal shaft 15, is only separated from the bearing 18 by just sufficient space to contain the take-off gear 20. An assembly nut is screwed around the forward end of the external shaft 16, abutting against the bearing 18 to stop axial movement. This nut (shown in FIG. 2), which extends forwards from the bearing 18 and is disposed just above the take-off gear 20, is difficult to access, particularly because a seal support sleeve must be placed behind the bearing 17 immediately in front of the nut. The sleeve overlaps the forward end of the external shaft 16 and a portion of the internal shaft 15 to create a seal between them. Disassembly of the high pressure shaft 16 for maintenance is usually only possible after disassembling the machine in front of the nut, and particularly removing the fan 6, so that the nut can be reached and unscrewed.
French patent 2 783 579 describes an assembly by which the nut can be unscrewed from behind by using a special tool inserted into the annular space between the shafts. This is more convenient despite the fact that this space is narrow, since there is then no need for any major disassembly of elements of the machine for removing the high pressure shaft, but in that patent the nut is screwed inside the external shaft whereas it is screwed around the shaft in more conventional designs, which changes the entire design of the forward end of the shaft and can introduce other disadvantages. In the present invention, another solution is proposed to provide easy access from the rear part of the machine to the nut retaining the external shaft 16 without making any major change to the design. A few modifications to parts of the machine surrounding the nut remain necessary, but it will be seen that they are mainly concerned with sealing and take-off devices, for which the design may be altered more easily.